Changing emission light overtime Blender 2.8
Hover your mouse over the emission strength and hit I. The property will turn yellow, meaning that you created a keyframe. Then go to a different frame, change the strength and it will turn green (meaning no keyframe) then hit I again hovering over that strength bar. It will turn yellow and you've got yourself a basic strength changing emission shader.

Note that this works for most properties in blender. That is how you can make transformations over time.